1. Field of the Invention
The present application relates to methods and apparatus for power harvesting in open drain transmitter configurations. The application further relates to but is not limited to methods and apparatus for power harvesting for HDMI (high-definition multimedia interface) operation.
2. Discussion of the Related Art
In open drain transmitters, such as those in HDMI operation, a receiver provides a common mode bias voltage across receiver terminating resistors to an open drain of a transmitter. FIG. 1 shows an example of a typical open drain transmitter/receiver. A transmitter 110 comprises a differential pair of open drain transistors 111 which transmit differential data over lines TX− and TX+ to a receiver 120 over the channel 130. The transistors 111 draw current from a voltage source 121 of the receiver and a voltage drop across terminating resistors 123 and 122 is used to determine the transmitted data. A current source 112 is used to determine the current drawn from the receiver which may be in accordance with transmitter protocol. Typically the current is fed back to the receiver via a ground path 113 coupled between the transmitter 110 and receiver 120.
Recently, some approaches have attempted to harvest this HDMI current to provide power to the transmitter. Using power harvested from the HDMI current would negate the need for an additional power source for the transmitter. However because a pre-amplifier is used to switch the data signal passed by known transmitters, the harvested current requires a voltage boost to maintain a suitable voltage difference for the preamplifier. Level shifters within the pre-amplifier produces speed limits on the transmitter output.
An example of the known HDMI current harvesting is shown in FIG. 2. FIG. 2 shows an open drain differential pair of transistors NL0 and NL1. The gates of NL0 and NL1 receive differential data from a pre-amplifier 220 and the drains of the transistors NL0 and NL1 output data to pads TX− and TX+. Data is input to the pre-amplifier from a serializer 210. A drive current Idr through the differential transistors NL0 and NL1 is determined by a current sensor/control block 241 that receives a bias signal. The current sensor/control block 241 forms part of a voltage recovery circuit 240 which recovers the power in the drive current through use of a voltage regulator 243 to provide a supply to the serializer 210, preamplifier 220 and other circuitry.
In order to switch the transistors NL0 and NL1, the pre-amplifier 220 requires a supply higher than a voltage regulated from the drive current. A voltage booster 242 is required to provide the pre-amplifier 220 with a boosted voltage.
The circuit of FIG. 2 cannot take full advantage of power harvesting due to the additional power used by the voltage booster 242. Additionally the voltage booster 242 produces speed limits on the transmitter output.
Therefore a need exists for power harvesting that addresses the abovementioned limitations.